1. Technical Field
The present invention relates to a detection apparatus and method, in which images of an adhered substance such as raindrops on a planar transparent member such as a windshield are captured, and detection of adhered substance is conducted based on the captured images.
2. Description of the Background Art
JP-2010-204059-A discloses a raindrop detector that can identify raindrops by capturing P-polarized and S-polarized images using an image capturing device. The S-polarized images and the P-polarized images can be captured as follows: a light source disposed in a raindrop-detector-equipped vehicle emits parallel light to a windshield (i.e., transparent plane) of the detector-equipped vehicle by setting the Brewster's angle as the incidence angle, and the image capturing device receives light reflected from the windshield, by which the S-polarized images and the P-polarized images can be captured. Then, based on a difference of reflection ratio between the captured S-polarized images and the P-polarized images, it is determined whether raindrops are on the windshield.
As noted, the parallel light is emitted to the windshield by setting the Brewster's angle as the incidence angle. Under such setting, if raindrops do not adhere on an outer face of the windshield, which is an opposite face of the windshield by which the parallel light coming from the light source strikes the windshield, only the S-polarized light is reflected at the outer face of the windshield. Therefore, a difference of reflection ratio between the S-polarized images and the P-polarized images captured by the image capturing device becomes great.
In contrast, if raindrops adhere on the outer face of the windshield, the S-polarized light and also the P-polarized light are reflect at the outer face of the windshield. Therefore, a difference of reflection ratio between the S-polarized images and the P-polarized images captured by the image capturing device becomes small. The raindrop detector of JP-2010-204059-A uses takes advantage of this feature to identify an image area having a small reflection ratio as an area with a raindrop.
However, light coming from an object disposed inside the detector-equipped vehicle may regularly be reflected at an inner face of the windshield, which is a face of the windshield from which the parallel light coming from the light source enters, and may be projected on the windshield as a ghost image. Such ghost image may be captured by the image capturing device with images of raindrops on the outer face of the windshield. Such ghost image, generated by regular reflection light, has greater light intensity. Therefore, the ghost image becomes ambient light, which adversely affects the precision of the raindrop detector of JP-2010-204059-A.
Further, when the light source emits light toward the windshield, before the light reaches the outer face of the windshield, the light coming from the light source may regularly be reflected at the inner face of the windshield as regular reflection light having greater light intensity. Such reflection light also becomes ambient light, which adversely affects the precision of the raindrop detector of JP-2010-204059-A.
Such problem may occur when any type of substances including raindrops adhere on a face of the transparent plane, which is an opposite face of the transparent plane that the light coming from the light source enters.